Commercial cucumber (Cucumis sativus) production can suffer from a variety of diseases. Yellowing viruses cause one such disease and may cause significant economic damage in cucumber production. The family of closteroviruses (Closteroviridae), which forms the highest taxonomic cluster of Yellowing viruses affecting cucumber, comprises more than 30 flexuous and filamentous, insect-transmitted plant viruses. The family consists of three genera, of which the whitefly-transmitted Crinivirus genus comprises species that are of particular concern to cucumber. This genus includes inter alia the species Cucurbit yellow stunting disorder virus (CYSDV), Lettuce infectious yellows virus (LIYV) and Beet pseudo-yellows virus (BPYV). CYSDV and BPYV pose the biggest threats to cucumber growers.
The viruses normally reside in the insect vector and are transmitted by the feeding activity of the insects on the plant. Cucumber closterovirus may therefore be controlled by means of application of insecticides. Preferably however, control of closterovirus in agriculture and horticulture is achieved by providing virus resistant cultivars of the host plants. At present, at least two quantitative trait loci or QTLs in the genome of resistant cucumber varieties are recognized as being associated with closterovirus resistance in cucumber (vide WO 02/22836). Proper introgression of the genetic material associated with closterovirus resistance into offspring plant lines may thus be monitored by detecting specific markers linked with these QTLs. Knowledge of the QTLs may therefore assist in the breeding of closterovirus resistant cultivars.
Another important disease that affects commercial cucumber production is cucumber powdery mildew (PM). PM in cucurbits may be caused by the fungi S. fuliginea and E. cichoracearum. The disease is widespread and can occur year-round. Symptoms start as small spots of a fine white fungal thread on the surface of infected leaves, which spots may grow out and eventually cover stems and foliage with a white, powdery mass of spores and hyphae. Severe infection results in discoloring and loss of leaves, with a concomitant reduction in the number and size of the fruits. Although a number of fungicides are effective against powdery mildew, resistance of the fungus towards the chemicals is emerging.
Various cucumber strains exhibit some level of resistance to powdery mildew. Such strains include for instance the Indian wild cucumber accession PI 197088, and accessions PI 200815, PI 200818, as well as the cultivars Natsufushinari and Asomidori (Morishita et al. 2003). Also various genes associated with resistance to PM are known (Fanourakis, 1984; Hujieda and Akiya, 1962; Kooistra, 1968, 1971; Shanmugasundarum et al., 1971, 1972), including pm-1 and pm-2 in Natsufushinari, pm-3 in PI 200815 and PI 200818 and “pm-h” in the cultigen Wis. SMR 18 (gene “pm-h” is without prejudice to the pm locus indicated by pm-h herein below). Although a few commercial cucumber varieties are available with partial tolerance to PM, most commercial growers still rely on the use of foliar fungicides. This is in part due to the fact that PM resistance is difficult to introduce into lines that exhibit other types of resistance. For instance, it has occurred to breeders as a disadvantage that lines acquiring PM resistance alleles become susceptible to closterovirus and vice versa, whereas previously they were not. In fact, crossing experiments have thus far not resulted in cucumber lines that exhibit resistance to both PM and closteroviruses. The beneficial double resistant recombinants have thus far not been attained. This is surprising, since by and large the merging of different traits into a single genome may be accomplished by relatively straightforward breeding procedures.
It is an object of the present invention to provide cucumber plants which exhibit resistant to viruses of the family of closteroviruses that affect cucumber, in particular BPYV and CYSDV, and which plant in addition thereto exhibits resistance to cucumber powdery mildew brought about by the fungi S. fuliginea and/or E. cichoracearum. 